The ability to image surfaces at atomically precise levels stems from the capabilities of the STM. While there have been many different implementations since its invention in 1982, the fundamental operating principle of an STM is as follows. A conducting tip—tungsten or platinum-iridium—is prepared so as to have a nanoscopic portion of the tip that allows electrons to tunnel to or from the tip and a sample. The tip is brought in close proximity (e.g., within a few nm) to a surface of a sample. Due to the principle of quantum electron tunneling, a current flows across the gap between tip and the sample when a bias voltage is applied between the tip and the sample. The bias voltage applied between the tip and the sample can be either polarity. If the sample is negatively biased with respect to the tip, then electrons flow from the filled electronic states on the surface into the tip. If the sample is positively biased, then electrons flow from the tip into the empty electronic states of the surface. The magnitude of the bias voltage determines the surface states that are available to tunnel into or out of. Thus, the STM provides information about the electronic states as well as the topography of the surface of the sample.
The resulting current between the tip and the sample based on the applied bias voltage varies exponentially relative to the distance between the tip and the surface of the sample. Atomic-scale surface features cause a change in tip-sample distance and consequently in the current. A control system measures the current passed through a current pre-amplifier and adjusts the Z-axis tip position to compensate for the current variations. Thus, the tip vertical motion is proportional to the height of atomic-scale surface features, and the controller generates topographical information characterizing the surface. Often, the control system actuates a piezoelectric element to control movement of the tip up and down (i.e., z-direction) until the measured tunnel current matches a set point value, which is in the range of about 0.01 to about 100 nA. Piezoelectric elements are also commonly used to move the tip sideways (i.e., x-y directions) across the surface of the sample. As a result, topographic images of the surface can be generated by performing a raster scan of part of the surface.
In practice, poor control performance of the control system, particularly in the z-direction can result in unsafe decreases in the tip-sample gap and even a tip-sample crash. Such a crash between the tip and sample can cause irreversible damage to both the tip and the sample, adding to the operation costs. Even a less impactful crash can compromise the integrity of collected topographic information or result in errors in patterning when the STM is used in nano-lithography applications. In lithography applications, the STM operates at higher current, higher bias voltage and in some cases smaller tip-sample gap, accordingly a tip crash may be more likely and the consequences of a crash may be even greater than in surface characterization applications. Due to its general robustness and relatively easy implementation, proportional integral (PI) controllers have been used as in control systems in commercial STMs. However, current controllers have not been satisfactory in all respects.
These figures will be better understood by reference to the following Detailed Description.